- The interference patern is quite uniform with a nice Lorentzian profile for the rings.
- The mesurements from the different rings are very consistent.
- There is a very slight improvement in the FWHM when the aperture is stopped down to 15 mm.

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Spectro measurement in a collimated beam

Optical setup: Sun => etalon => Solex with 7 microns x 6 mm slit, fc = f i =200 mm, 2400 l/mm grating, ASI 290, 0.0471 A/pixel dispersion

Top image : spectrum transmitted by the etalon.
Bottom image : solar spectrum.



The transmission profile (= transmitted spectrum/solar spectrum) consists in textbook Lorentzian curves (curve fitting with Fityk).





FSR increases with the order of interference (as expected). Fringe #4 is on Ha:


 
The values for Ha (fringe #4) are :
FWHM = 0.57 A and FSR = 11.8 A.

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Spectro measurement in a f/35 telecentric beam

Optical setup: Sun => C8 EdgeHD => f/35 telecentric => etalon => Solex with 7 microns x 6 mm slit, fc = f i =200 mm, 2400 l/mm grating, ASI 290, 0.0471 A/pixel dispersion





Curve-fitting done on etalon transmission = transmitted spectrum/solar spectrum :

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Peak transmission

Optical setup: Sun => C8 EdgeHD => f/35 telecentric => etalon => Solex with 7 microns x 6 mm slit, fc = f i =200 mm, 2400 l/mm grating, IMX585, 0.0471 A/pixel dispersion
10 May 2025 - Sun elevation = 41°


Comparison of the intensity of the lines of the solar spectrum with / without etalon allows to estimate the peak transmissionn to about 59% at f/35 (including the ERF). 
It is difficult to estimate the uncertainty of the measurement.The spectra without/with the etalon were taken at 4 min interval. Sky transparency might have changed in between.

By comparison, a second etalon without ERF was measured 20 min later at 68% without the ERF. This is consistent with the 10% absorption of the RG630 with 3 mm thickness.

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Test of the etalon in Ha collimated beam

Optical setup: Ha lamp => 3 mm aperture stop => 55 mm f/8 collimating fluo doublet => etalon in collimated light (equivalent to f/146 ratio) => Nikon Z6 with 105 mm f/2.8 S macro lens



Comments:
This test is quite demanding and shows any on uniformity of the etalon. The etalon under test is clearly very uniform. Traces of the manufacturing process are visible.

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Test of the etalon in Ha collimated beam - Impact of machanical compression

Optical setup: Ha lamp => 3 mm aperture stop => ZWO80/600 => etalon in collimated light (equivalent to f/200 ratio) => Nikon Z7II with 105 mm f/2.8 S macro lens at f/3.2



Increasing compression is applied to the rim of the etalon (through a PTFE washer):